Habitable planet

• Uninhabitable Planet - none of the known planets and natural satellites in the solar system as well as those detected orbiting stars other than the Sun seems likely to support life.
• * Lifeless Planet
• ** Lifeless Unterraformable Planet - An uninhabitable planet which cannot conceivably be altered so that its surface conditions would be capable of sustaining life. Because of its surface gravity, surface atmospheric pressure, and radiation field, Jupiter is not expected to be terraformable.
• ** Lifeless Terraformable Planet - perhaps some of the known worlds could, by means of an effort unworkable at present, be terraformed; that is, have its surface conditions changed to permit the seeding with plants which would produce a biosphere that will sustain human life.
• * Terraformable Planet with a Minimal Biosphere - this includes any planet with the least amount of life on it, down to microorganisms. Here the amount of life is too small to produce an atmosphere sufficient to breathe without an oxygen tank. Perhaps Mars is one of these.
• * Unterraformable Planet - these have a biosphere inimical to human life, e.g., one not based on sunlight, water, oxygen, and carbon-dioxide, or one that produces toxins that no countermeasure could be devised. Here it could not be terraformed.
• ** Uninhabited Planet with a Biosphere Inimical to Human Life - this has also not yet been discovered.
• ** Inhabited Planet with a Biosphere Inimical to Human Life. - These inhabitants would require a different set of physical and biological requirements. Example: Hal Clement's A Question of Gravity.
• Marginally Inhabitable Planet - This is the setting that some science-fiction stories made use of. Some special equipment would be needed to keep out either chemical toxins or microorganisms that would make people ill, or concentrate the oxygen in the atmosphere.
• * Uninhabited Marginally Inhabitable Planet - There's no aliens on it and it can be settled with some difficulty. Example: Weinbaum's Venus.
• * Inhabited Marginally Inhabitable Planet - There are aliens on it but terrans could settle it, and may have to compete with them, but they would be at a disadvantage. Example: Heinlein's Red Planet.
• * Inhabited Terraformable Marginally Inhabitable Planet - There are aliens on it and if terrans alter the biosphere to their liking, they may die.
• Completely Inhabitable Planet - This is the setting the science-fiction stories, now called "space opera" required and revelled in.
• * Uninhabited Completely Inhabitable Planet - There's no aliens on it and it can be readily settled. In the human context of myths, this resembles some Americans' dreams.
• * Inhabited Completely Inhabitable Planet - There are aliens on it but terrans can settle it and may have to compete with them. This mythologically resembles the New World as discovered by Europeans.

In the 1960s, the CIA were concerned that the Soviets might get to find a habitable planet in this solar system. Stephen Dole did a report which was popularized by Isaac Asimov under the title Planets for Man. Since that time, both Venus and Mars were determined to not be habitable, so general interest has shifted to the concept of terraforming these worlds.